The largest single application for rubber is in vehicle tyre. The principal rubbers used are the hydrocarbon polymers Natural Rubber (NR), Styrene Butadiene Rubbers (SBR) and Polybutadiene Rubbers (BR). During processing the polymer molecules are vulcanised i.e. cross-linked by sulphur atoms. The formation of cross-links enhances the mechanical properties of the rubber but renders it unsuitable for easy processing. Throughout this application, vulcanised rubber is taken to mean rubber cross-linked by sulphur.
Waste rubber materials, such as vehicle tyres, present a significant environmental problem.
Currently, the European Union scraps a total of nearly 2×106 tonnes of tyres per year of which 23% are retreaded and 46% disposed to landfill. In the UK, it is estimated that approximately 100000 tonnes of tyres are disposed of each year, mainly in landfill sites. In the USA, 300 million tyres are disposed to landfill per annum. In New York State alone, 12 million tyres are discarded per annum, which represents in excess of three million barrels of oil in energy equivalent being discarded.
The United Nations and EU have warned that waste rubber is becoming a significant environmental problem worldwide. The EC Landfill Directive (1999/31/EC) has advocated the banning of disposal to landfill by 2003 for whole tyres and 2006 for shredded tyres.
Currently, alternative means of disposal of waste rubber, in particular waste tyres, include various recycling methods and burning of e.g. in cement kilns. However, burning of rubber materials such as tyres can produce significant pollutants including dioxins.
Life cycle analysis shows that only a small fraction of the energy used in manufacturing tyres is recovered on combustion.
Conventional recycling methods include mechanical, thermo-mechanical, cryomechanical, microwave and ultrasonic methods. Chemical recycling methods include reclamation using organic disulphides, mercaptans and inorganic compounds. However, rubber produced using these methods has poor mechanical properties. There is also pyrolysis of waste rubber to oils and carbon black. Alternative methods of recycling include employing reclaimed rubber that is blended with Low Density Polyethylene (LDPE) to produce mixed polymer elastomeric material. However, rubber produced using these methods has poor mechanical properties.
Rubber crumb is also used in roads, playparks, running tracks and equestrian surfaces. However none of these applications come close to using the large quantities of waste rubber available.
A recent biotechnological approach to recycling rubber involves elastomer recycling to the viscoelastic state by removal of sulphur by the thermophilic bacterium Sulfolobus. However, the use of a thermophile at temperatures in the region of 70° carries a large energy cost penalty and generates highly acidic environments with production of sulphuric acid which can lead to reprocessing problems. DE04042009 describes the surface treatment of rubber crumb by suspensions of chemolithotrophic Sulphur-oxidising bacteria allowing revulcanisation to take place when mixed with new (virgin) rubber stock and devulcanisation of comminuted rubber scrap by similar suspensions of chemolithotrophs to produce viscoelastic rubber and sulphuric acid. However, the presence of sulphuric acid in the reaction mixture would be detrimental to revulcanisation and to the quality of any reprocessed rubber.
As current methods of disposal of waste rubber are environmentally unacceptable and conventional methods of recycling produce low quality rubber materials, there remains a need for a method of recycling rubber materials which produces rubber of high quality and reduces environmentally unacceptable consequences.